Pit of bones An analysis of skulls found in a Spanish cave reveals the oldest-known humans to have Neanderthal-like features lived about 430,000 years ago.

The skulls however also show characteristics associated with an earlier, primitive human suggesting a more complex evolution of humans than previously thought, the international research team says.

The blend of characteristics also opens the debate as to whether these fossils mark a new species, although the researchers step back from making that claim in the paper published today in Science .

The findings come from excavations at a cave in northern Spain, Sima de los Huesos, (Pit of Bones) which contains the world's largest collection of ancient human fossils.

The study focuses on a collection of 17 fossil human skulls and comes 21 years after the presentation of the first three skulls from the site was made in 1993.

Co-author Dr Lee Arnold, a geochronologist at the University of Adelaide who co-ordinated the dating element of the study, says the latest dating clears up controversy over the age of the Sima fossils.

A previous study had placed the fossils as more than 530,000 years old, but this age was incompatible with the fossils' morphology and genetic evidence, says Arnold.

To clear up the dispute, the team used six dating techniques that could be used on different sample types such as sediment, bones and teeth, other cave structures and accretions of the skulls.

"When we focused on [four of the six] techniques they all converged on 430,000 years ago," he says.

Arnold says their dating provides a solid age for the first appearance of Neanderthal features in the fossil record.

Mosaic of evolution

He says one of the aims of the study was to also test a theory, known as the accretion model, on how Neanderthals evolved.

This theory, says Arnold, proposes that Neanderthal features did not emerge at the same time but that a "mosaic of evolution" occurred so that different features developed separately from each other.

He says the analysis of the 17 skulls in this paper supports this view as Neanderthal features are present in the jaws and teeth, while the upper skulls are like those of the earlier, more primitive Homo heidelbergensis.

"Heidelbergensis is defined by the jaw and at Sima the jaw is completely different, so it isn't Heidelbergensis but it isn't classic Neanderthal either," Arnold says.

"The paper [suggests] it might be a different species but … it will be debated and this is the opening of this process.

"A picture is emerging of human evolution which is way more complex than has been considered over the past couple of decades." says Arnold.

Another key element, he says, is that the 17 fossil skulls show very strong homogeneity of traits, which contrasts with other archaeological sites in Europe from that timeframe (430,000 years ago) that have fossils with very different morphology.

"There are sites in southern Serbia about the same age but no Neanderthal features whatsoever are found in the fossils there," says Arnold.

"This suggests different types of human species were running around Europe at the same time."

Ceremonial burial ground

Arnold says the Sima site is "fascinating" and he has never seen a site with so many bones.

To date more than 6500 human fossils from at least 28 individuals have been recovered from Sima de los Huesos and their skeletons appear to be complete, although require reconstruction.

He says the Spanish excavators believe the site was a ceremonial burial ground and that the dead were taken to the edge of the vertical shaft and thrown down.

The lack of stone tools at the site and the presence of complete skeletons — albeit in many pieces — is consistent with this view, he says.

However, he says, there must have been an easier entrance for the ancient people and future work will be to identify how they arrived at the site.

"To get there you have to crawl through 500 metres of chambers and caves and abseil 14 metres down a vertical shaft and once you reach the pit of bones you are in there it is a really tight space."